Deodorizing and sterilizing in use of photocatalysts has been researched in various ways, and some is used actually. For example, WO 94/11092 discloses an air treatment method using photocatalysts under indoor illumination. Japanese Unexamined Patent (KOKAI) Publication Heisei No. 7-102,678 discloses a method for preventing intra-hospital infection using photocatalysts. Each method uses oxide semiconductors such as titanium oxide as a photocatalyst, which needs ultraviolet ray having wavelength 400 nm or less as the excitation light.
Meanwhile, solar light or artificial light used as the exciting light source contains visible light in addition to the ultraviolet ray. The photocatalyst made of an oxide semiconductor such as titanium oxide, however, does not use the visible light, and efficiency of energy conversion is very poor.
Due to remarkable development of the chemical industry, various chemical substances exist widely in living environments, and human bodies are adversely and invisibly affected where those chemical substances exist in living space even in a quite small amount. For example, formaldehyde is utilized as a sterilant or insecticide and used as an adhesive for wall paper, furniture, and clothes. In a living environment with a poor air ventilation, the formaldehyde evaporated from the wall paper or the like, for example, may adversely affect human bodies, particularly, bodies of infants.
To remove organic compounds such as formaldehyde or the like existing in the living environment in a small amount, an absorbent or the like may be used. Such absorbent, however, cannot be said as adequately useful in terms of period for use and absorbing performance.
Therefore, a new technology has been expected to further effectively remove formaldehyde or the like existing in a living environment in a very small amount.
Tar produced from tobaccos may adhere to, e.g., walls, lamps, and windows and impair those appearances. Such tar had been removed so far by washing or the like, but such works are too laborious. Therefore, a new technology has been expected to easily remove such tar or the like adhering to, e.g., walls, lamps, and windows.
It is a first object of the invention to provide a new photocatalyst capable of using visible light.
It is a second object of the invention to provide a photocatalyst rendering some organic compounds such as a formaldehyde harmless and a method for rendering such organic compounds such as a formaldehyde harmless using the photocatalyst.
It is a third object of the invention to provide a method for removing various substances including organic materials such as tar upon photo-decomposition using the above photocatalyst.